Speed measurements, sound direct

When an isotropic material is subjected to planar shock compression, it experiences a relatively large compressive strain in the direction of the shock propagation, but zero strain in the two lateral directions. Any real planar shock has a limited lateral extent, of course. Nevertheless, the finite lateral dimensions can affect the uniaxial strain nature of a planar shock only after the edge effects have had time to propagate from a lateral boundary to the point in question. Edge effects travel at the speed of sound in the compressed material. Measurements taken before the arrival of edge effects are the same as if the lateral dimensions were infinite, and such early measurements are crucial to shock-compression science. It is the independence of lateral dimensions which so greatly simplifies the translation of planar shock-wave experimental data into fundamental material property information. 

Figure 8. Comparisons of the specific volumes of water determined from sound speed measurements (31) and direct measurements (106J...

Since the compressibility is proportional to the pressure derivative of the volume, any experiment that establishes the P-V-T relation of a gas with sufficient accuracy also yields data for the isothermal compressibility. For obtaining the adiabatic compressibility from the P-V-T relation, some additional information is necessary see section (c). Tor instance specific heat data in the perfect gas slate of the substance considered. A more direct way of determining ihe adiabatic compressibility is by measuring the speed of sound i1. the two quantities being related by... 

Speed of sound is thus directly and analytically related to bulk-material properties and becomes a useful measure of fat structure. 

Acoustic intensity (/) is a measure of the amount of energy transmitted to the liquid and is defined as the rate at which the acoustic energy passes across a unit area perpendicular to the direction of the propagating sound wave. It can be shown that the acoustic intensity is proportional to the square of the amplitude (Pa) of the acoustic wave divided by the density of the liquid (p) and the speed of sound in the liquid (c) (e.g., c = 1500m/sec for water) ° ... 

Schubauer s work on turbulence and airflow, and the development of instruments for measuring these phenomena, was vital to the development of the modem high-speed aircraft. In the 1950s, he studied the accuracy of the hot-wire anemometer at speeds up to twice the speed of sound. This instmment was previously a basis in aerodynamic research at subsonic speeds, but it was not known whether it could also be used at supersonic speeds. Schubauer was elected to the National Academy of Engineering in 1980 for the discoveiy of self-exited oscillations in laminar boimdaiy layers, giving a new direction for further inquiry into the origin of turbulent flows . He furthermore was the recipient of the 1988 Fluid Dynamics Prize from the American Physical Society APS. 

Another version of the multiple echo technique (172,173) that has been used to make very accurate sound speed measurements in pol5miers makes use of a single transducer bonded directly to the specimen (110). Pulses are reflected from the opposite face and return as echoes to the front face. By adjusting the pulse repetition rate, an echo from a later pulse can be made to overlap a multiple echo from an earlier pulse. When the repetition rate is adjusted to obtain an in-phase condition, constructive interference will result in a maximum amplitude in the superimposed signals. A value of sound speed accurate to 0.05% can be obtained by observing several repetition rates that 5deld an in-phase condition at the transducer resonant frequency and by repeating the measurements at some other frequency, eg, 10% below resonance, and on other specimens that differ only in thickness. 

An ultrasonic transducer can be made to produce a pulse which will propagate in a straight line into the sodium. If it encounters an inter ce with a region in which the speed of sound is different - a solid object, for example - it will be reflected. If the interface is flat and smooth and normal to the direction of propagation it will be reflected back to the transducer, which can be arranged to detect it. The time interval between transmission and reception can be measured and, if the speed of sound in sodium is known, the distance from the transducer to the object can be deduced. 

The acoustic pressure amplitude determines the growth of a cavitation bubble and consequently the chemical effects upon collapse. The amplitude of the pressure wave can be measured directly with a hydrophone or calculated using a calorimetric method [128, 129], by which it is possible to determine the ultrasound power Qus) that is transferred to the liquid. With the ultrasound power, the density of the liquid (/ ), the speed of sound in the medium v), and the surface area of the ultrasound source (Ays), the acoustic amplitude can be calculated according to Eq. (3). The ultrasound intensity is the power input divided by the surface area of the source [130]. 

It is not an easy task to measure directly isothermal coefficients of compressibility with the precision necessary to give reliable apparent molar compressibilities at low pressure. However, it is possible through speed of sound measurements using modern instrumentation to obtain very precise values of a related quantity termed the isentropic coefficient of compressibility. 

As a means to compare directly 8p and 8u, we now introduce the pressure changes that are associated with velocity and density changes. It will be convenient to relate the fluid velocity to the sound speed as a measure of the magnitude of the velocity. By definition, the sound speed a is a property of the fluid defined as... 

As the design matures, the direct measurement of the acoustic properties becomes necessary. These properties include the longitudinal wave speed, the coefficient of attenuation and the acoustic impedance, which can be obtained from measurements of the reflection and transmission of sound by the material. Two acoustic techniques are available for these measurements, the impedance tube and the panel test. 

It was estimated from wave data that only in a zone around the margins of the Sound where the water is less than 18 m deep are the particle velocities of waves a significant fraction of the tidal stream speed. Direct evidence of excitation of sediment by waves in this zone is found in turbidity measurements and in the structure of surficial sediment layers. For example. Fig. 11 shows a turbidity track made from deep to shallow water in an area where the bottom is mud and the tidal stream weak. There is resuspension of mud through the water column where the depth is less than about half the wavelength of the waves present at the time the track... 

Elastic constants of bulk crystalline materials are commonly measured by means of the ultrasound method where sound velcities in specific crystallographic orientations are monitored (( )). For example, the case of a cubic crystal for which the longitudinal wave speed along the [100] direction is a/cii/p, where p is the density of the crystal. Likewise, the shear wave speed along the [100] direction is p. The speed of the longitu-... 

The first Brillouin scattering measurements on the smectic A phase were made by Liao et al. [33] who confirmed the predictions of the propagating mode structure in both smectic A and smectic B phases. The first and second sound velocities were measured for racemic P-methylbutyl p-[(p -methoxy-benzylidene)amino]cinnamate, and they showed that the first sound speed is almost isotropic with a minimum in the off-symme-try direction, with the speed of second sound being extremely anisotropic. The data are well described by theory. No 2nd sound measurements were made for (])>45°, indicating very anisotropic damping that was at-... 

If sweeping is used then other frictors must also be considered. The speed with which the probe is swept across the subarea must be uniform, at about 300 mm/sec, and the area should be covered by a whole number of sweeps with an equal separation between sweep lines. Care must be taken that excessive dwell time does not occur at the edges of the subarea when the probe s direction of sweep is reversed. The operator must also be carefiil that his or her body does not influence the measurements by obscuring sound entering the measurement area as he or she sweeps. 

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